ES2471143T3 - High durability sheet for the manufacture of banknotes - Google Patents

Description

DESCRIPTION

High durability sheet for the manufacture of banknotes

The present invention relates to security papers and more particularly, but not exclusively, to those intended for the manufacture of banknotes, as well as to a method of manufacturing such papers. Banknotes are exposed during their useful life to numerous manipulations and risks of dirt. In order to confer high durability, the fibrous substrate is protected by one or more layers of a protective coating that has a barrier effect against dirt.

The choice of the protective coating is complex, since it must at the same time ensure a barrier function and not harm the printability of the paper too much during the manufacture of the bills.

Numerous protective coatings have already been disclosed in the prior art.

EP 514455 B1 thus discloses the improvement in the resistance to the circulation of banknotes by impregnating the fibrous substrate, before printing, of a composition containing one or more binders selected for their mechanical characteristics.

According to this patent, at least one of the faces of the fibrous substrate is treated with a composition comprising one or several fillers selected from the mineral fillers and / or the plastic coating pigments, and at least one elastomer binder in an amount greater than 25 parts per 100 parts of dry weight load. The elastomeric ligand can be selected from the group consisting of the aqueous dispersions of polyurethane, acrylate copolymer, styrene butadiene copolymer, optionally carboxylated, polymers of which one of the monomers is acrylonitrile or isoprene or neoprene, or mixtures thereof . Preferably, a polyurethane is used. The load is preferably mineral, selected from the silicas or kaolins. To deposit the coating layer, an air jet coating machine is preferred, in order to respect the watermark.

The papers made using the teaching of this patent are overall satisfactory since they have a good resistance to abrasion of the printed paper, as well as a satisfactory resistance to dirt against greases and pigments in liquid medium. However, its resistance against grease and minerals in a powdery medium could be improved.

Application US 2007/0017647 A1 discloses a paper intended for the manufacture of banknotes, which has an improved dirt resistance. According to the teaching of this application, the protective coating comprises at least two successively applied layers, of which a base layer that is in contact with the fibrous substrate and closes its pores and an outer layer that protects the substrate from physical influences and chemical The base layer may comprise polyurethane while the outer layer is preferably photocrosslinkable. The application of the base layer can be carried out by flexographic printing. In one example, the base layer is an aqueous dispersion of a mixture of acrylic styrene polyurethane, while the outer layer is a styrene-acrylic aqueous dispersion. When the outer layer is crosslinkable by drying, it may comprise an aqueous dispersion, preferably without polyurethane, for example based on acrylic styrene.

WO 96/28610 discloses the application of a liquid polyurethane-based composition on a fibrous substrate, in order to form a protective coating.

US 2006/0127649 indicates that the application of a liquid composition allows to obtain a better resistance to dirt, but does not improve the mechanical stability of the substrate.

Application US 2004/0023008 A1 discloses a security paper intended for the manufacture of banknotes. The paper comprises a fibrous substrate that is coated on at least one of its faces with a transparent or translucent protective layer comprising colloidal silica and at least one transparent or translucent elastomeric binder, comprising for example polyurethane.

Publication WO 2008/054580 A1 discloses a method for conferring a resistance to moisture and dirt on a fibrous substrate, which consists in impregnating the substrate on two opposite faces with an aqueous composition comprising one or more thermoplastic resins selected from the resins which have an ester bond, polyurethane resins, functionalized polyurethane and the copolymers and mixtures of these resins. More particularly, the resins can be selected from the aqueous dispersions of a polymer, which in particular has resin particles selected from polyurethane, polyether-urethane, urethane-acrylic resins and mixtures thereof. In a preferred embodiment, the applied composition comprises, in addition to the aqueous polymer dispersion, one or more pigments such as titanium dioxide and possibly one or more crosslinking agents. US 1499235 discloses the improvement in the resistance to circulation of a banknote by impregnating the fibrous substrate with a latex comprising a copolymer based on carboxylated styrene and butadiene.

EP 815321 B1 discloses a paper intended for the manufacture of banknotes, in which an aqueous dispersion of polyurethane is deposited on the fibrous substrate, this dispersion of charge polyurethane being devoid and transparent.

On the other hand, high durability papers are known in which a fibrous substrate is coated with a PET film bonded to the substrate by a polyurethane-based adhesive, this PET film being in turn coated with an acrylic varnish, such as It is described in publications EP 1545902, EP 1854641 and EP 1827823.

The known solutions are not entirely satisfactory in terms of manufacturing cost and / or performance. US 3288632 discloses a process for manufacturing glossy paper, intended for the manufacture of magazines or books, of high opacity. This procedure comprises the successive application of two highly pigmented layers. The teaching of this patent cannot be applied to papers intended for the manufacture of banknotes, for which a high opacity of the layer or layers is not sought due to the presence of securities that must be seen in transmitted or reflected light , for example filigree type, prints with reference point or security threads.

EP 1783273 discloses an improved durability safety paper, in which a composition containing a fluorinated resin is applied on the surface. This composition can be mixed with additional compounds.

The aim of the invention is to make a paper of high durability, especially intended for the manufacture of banknotes, which offers a good resistance to circulation, especially a good resistance to powdery dirt in dry medium, to friction in wet medium and to dirt in a humid environment, capable of satisfying all manufacturing obligations, especially printing, on the other hand. The invention also aims to produce a high durability paper that can be manufactured at a relatively low cost.

The object of the invention is, according to one of its first aspects, a sheet of high durability paper according to claim 1.

The external printability layer is non-fluorinated, that is, it is devoid of fluorinated resins, especially resins comprising the C-F, CF2 or CF3 groups.

The external printability layer is intended to receive at least one print, for example chalcographic.

Thus, the fibrous substrate does not print directly. Printing is done on the protective coating.

Thanks to the invention, it is possible to benefit from a role of increased circulation resistance, and in particular that it exhibits at the same time a good resistance to powdery dirt in dry medium, a good resistance of the prints to friction and abrasion and a good resistance to dirt in a humid environment, while being compatible with the printing techniques used to print banknotes.

The invention allows not to use a film glued or produced by extrusion, the embodiment of which significantly increases the manufacturing cost of the sheets produced. The invention allows conventional coating, impregnation or coating means, commonly used in the paper industry, to be used to deposit the base layer and the external printability layer.

The coating according to the invention, and in particular the base layer, provides protection of the fibrous substrate and an improved resistance to dirt. In contrast to most of the layers applied by coating, impregnation or coating in stationery, which are opacifying pigmented layers to improve the printability and appearance of the surface, the layers of the coating according to the invention are relatively little or not pigmented, and they are more similar in composition to varnishes generally applied by printing procedures, therefore usually outside the paper machine and after paper printing.

The total thickness of the protective coating is preferably greater than or equal to 3 pm, especially between 5 and 25 pm. The weight of the protective coating is preferably less than or equal to 25 g / m2 per side.

The external printability layer advantageously comprises a mineral filler and / or polymeric particles, which allows the printability to be improved. The mineral and / or polymeric particle charge are preferably in a mass quantity, in dry weight, with respect to the total weight of the external printability layer less than 50%, better between 5% and 25%, even better between 6% and 10%. The mineral filler advantageously comprises silica and / or kaolin and / or talc and / or calcium carbonate. Preferably, the mineral and / or polymeric particle charge is non-opacifying thanks to its natural transparency and / or micron dimensions, especially less than 10 | im.

The external printability layer comprises, in a particular embodiment of the invention, a load of polymeric particles, preferably organic, for example a polyethylene, polyamide, polypropylene, an acrylic or styrenic polymer powder.

The external printability layer is preferably transparent or translucent.

The external printability layer preferably comprises at least 25%, better at least 30% and more preferably at least 50% dry weight of polyurethane and according to a particularly advantageous variant, the external printability layer comprises between 25% and 50% dry weight polyurethane, preferably mixed with an acrylic compound, especially with substantially equal amounts of polyurethane and acrylic compound.

The base layer comprises a polyolefin, preferably polyethylene (PE) or polyvinylidene chloride (PVDC). In a variant, the base layer comprises polyurethane, acrylic polymer, styrenic polymer or fluorinated polymer, for example of the polyvinylidene fluoride (PVDF) type. The base layer may be devoid of fillers, in particular mineral fillers or charges of polymer particles. Preferably, the polyethylene is a modified polyethylene, especially comprising acid groups and preferably carboxylic acid groups. The thickness of the base layer, per face, ranges from 1 to 24 | im. The weight of the base layer ranges, for example, from 1 to 24 g / m2 and preferably from 2 to 5 g / m2

The thickness of the external printability layer, per side, ranges from 1 to 24 | im. The weight of the outer layer ranges, for example, from 1 to 24 g / m2 and preferably from 2 to 5 g / m2.

The base layer is preferably transparent or translucent.

The coating may comprise only the two previous layers; In a variant, the coating may comprise one or several intermediate layers, in particular of an identical nature to one of the aforementioned layers. According to a particular case, at least one of the layers of said coating comprises a two-component system, called pro-polyurethane. In particular, said bicomponent system uses a polyaddition reaction between an isocyanate oligomer and a polyol oligomer. In particular, said two-component system makes it possible to synthesize in situ, that is to say on the substrate, a polyurethane.

According to a particular case, at least one of the layers of said coating comprises a crosslinkable polymer, in particular crosslinkable under the effect of ultraviolet (UV) radiation. According to a particular case, said crosslinkable polymer is a polyurethane.

Preferably, the deposition of the external printability layer is made online.

The deposit of the base layer and the deposit of the external printability layer preferably takes place on both sides of the substrate. In particular, any suitable coating system called two-sided coating (C-2-S) can be used to deposit said layers. Preferably, an air sheet coating system is used. The deposit can also be made, among others, by curtain coating, by pencil coating, by coating with the help of rollers, in particular pre-dosed, engraved or transfer, or also by immersion, by impregnation, by preparation of the surface or spray. Preferably, the fibrous substrate is coated, on each of its faces, by the coating.

Preferably, the fibrous substrate is impregnated with a binder that allows reducing its porosity, preferably based on polyvinyl alcohol (PVA). The PVA is dissolved in water at a height of 1% to 10% by weight, preferably between 3% and 6%, before impregnation of the paper substrate.

The fibrous substrate may incorporate at least one security element.

The object of the invention is also, according to another of its aspects, a method of manufacturing a high durability sheet according to claim 9.

The coating of the external printability layer can be carried out on the base layer directly, preferably after drying it.

According to an advantageous feature of the procedure, the base layer is deposited in line with the help of an impregnator.

Preferably, the fibrous substrate is pre-bonded to the deposit of the base layer, in particular prior to its passage through the above-mentioned impregnator. The gluing is preferably carried out with a PVA-based binder. Other binders may be considered as a complement or as a substitute for PVA, such as dispersions based on styrenic or acrylic polymers.

The base layer can be deposited in an amount ranging from 1 to 24 g / m2 and preferably between 2 and 4 g / m2 per face, in dry weight. The external printability layer can be deposited in an amount ranging from 1 to 24 g / m2 and preferably between 2 and 5 g / m2 per face, in dry weight.

The base layer may comprise a polymer selected from polyurethanes (PU), polyolefins, especially polyethylene, PVDC, acrylic polymers, styrene polymers and fluorinated polymers, in particular PVDF, as well as mixtures and copolymers thereof.

The external printability layer preferably comprises at least 50% dry weight polyurethane. The external printability layer may comprise at least 5% dry weight of a mineral filler, especially silica and / or kaolin and / or talc and / or calcium carbonate.

The external printability layer may comprise between 1 and 15% by dry weight of crosslinker, better between 1 and 3%.

The invention also discloses a method of manufacturing a banknote, in which a sheet of paper as defined above is printed on its outer printability layer, especially by chalcographic printing.

Fibrous substrate

The fibrous substrate comprises paper fibers and any additive suitable for manufacturing. In particular, the fibrous substrate of a mainly cellulosic nature may be reinforced by the addition of synthetic fibers in an amount between 1 and 50%, preferably between 5 and 15% by mass.

The fibrous substrate may comprise, when it is intended for the manufacture of banknotes, cellulosic fibers and more particularly cotton fibers. Other plant fibers of annual plants may enter the fibrous substrate composition.

The fibrous substrate may have one or more paper jets assembled in the wet state.

The fibrous substrate may have, as finished and dry paper, before gluing and depositing the protective coating, a weight between 20 and 120 g / m2 and a thickness between 30 and 180 | im. The fibrous substrate may comprise one or more watermarks, as well as one or more security elements.

Among the security elements that can be incorporated into the fibrous substrate, some are detectable with the naked eye, in daylight or with artificial light, without the use of a particular device. These security elements comprise, for example, colored fibers or strips, fully or partially printed or metallic threads. These security elements are called first level.

Other types of supplementary safety elements are detectable only with the help of a relatively simple apparatus, such as a lamp emitting in the ultraviolet (UV) or infrared (IR). These security elements comprise, for example, fibers, strips, bands, threads or particles. These safety elements may be visible to the naked eye or not, for example being luminescent under the illumination of a Wood lamp emitting at a wavelength of 365 nm. These security elements are called second level.

Other types of security elements require a more sophisticated detection device for detection. These security elements are, for example, capable of generating a specific signal when they are subjected, simultaneously or not, to one or several external excitation sources. Automatic signal detection allows the document to be authenticated, if necessary. These safety elements comprise, for example, tracers that are presented in the form of active materials, particles or fibers, capable of generating a specific signal when these tracers are subjected to an optoelectronic, electrical, magnetic or electromagnetic excitation. These security elements are called third level.

Reagents can also be incorporated into the fibrous substrate, for example, chemical or biochemical reagents, anti-counterfeit and / or authentication and / or identification that can react in particular respectively with at least one counterfeit and / or authentication agent and / or identification.

The security element (s) present within the fibrous substrate may have first, second or third level security characteristics.

Protective coating

Base layer

The base layer comprises any polymer adapted to confer barrier properties to the coating, in order to protect the fibrous substrate from dirt.

The barrier properties are equivalent to those of a plastic film. Thus, the sheet of paper according to the invention especially exhibits a dirt resistance equivalent to that of a fibrous substrate provided with a laminated, countercoated or thermoformed film.

The base layer is preferably present per side at a rate of 1 to 24 g / m2 in dry weight, better than 1 to 15 g / m2, even better at 2 to 5 g / m2.

The thickness per face of the base layer ranges, for example, from 1 to 24 | im, better from 1 to 15 | im, even better from 2 to 5 | im. The base layer is deposited on each face in a liquid state by coating or impregnation, preferably in line on the paper machine, preferably with the help of an impregnator.

The base layer is initially, preferably, an aqueous phase preparation, especially an emulsion or a dispersion.

The base layer is based on polyolefin or PVDC. In a variant, an acrylic, styrenic or fluorinated polymer, especially of the PVDF type, can be used.

Example of a base layer based on PU not covered by the invention (referred to as PU formula below)

Example of base layer based on polyolefin (called PE formula below)

Example of base layer based on PVDC (called PVDC formula below)

Pre-glued

Preferably, the fibrous substrate is pre-glued before coating or impregnating with the composition intended to form the base layer.

Pre-gluing is preferably carried out with a PVA-based binder. Polyvinyl alcohol is dissolved in water at a height of 1% to 10% by mass, preferably between 3% and 6%, before impregnating the paper substrate. Other binders may be considered as complement or replacement of PVA, such as dispersions based on styrenic or acrylic polymers, as mentioned above.

External printability layer

The outer layer is initially, preferably, an aqueous phase preparation, in particular an emulsion or a dispersion.

According to the invention, the external printability layer is based on PU.

The polyurethane is preferably applied in the form of an aqueous dispersion of polyurethane or propoliurethane particles.

The polyurethane may be present in the composition intended to form the outer layer in a mass amount between 40 and 100% with respect to the total weight of the composition.

The composition intended to form the outer layer may comprise a crosslinker selected from isocyanates, carbodiimides or aziridines. The crosslinker can be in a mass amount, in dry weight, between 1 and 15%, better between 1 and 3%, with respect to the total weight of the composition before coating.

The composition intended to form the outer layer also preferably comprises a mineral filler selected from silica, kaolin, talc or calcium carbonate.

The composition intended to form the outer layer also preferably comprises a load in a mass proportion in dry weight between 1 and 60%, better 3 and 10%, with respect to the total weight of the composition.

Example of external printability layer (same PU formula as before)

Coating media

To apply the base coat, an in-line impregnator is preferably used, as mentioned above, which in particular comprises pre-dosing rollers, engraved rollers, transfer rollers before dosing at the outlet.

To apply the external printability layer, a double-sided coating machine per air sheet is preferably used, for example of the TWINtM ABC type.

In addition, simultaneous coating patterns can be considered, for example, of the TWINTM ABC type, or with rollers, for example of the TWINTM sizer Gravure or TWINTM Sizer HSM type, to cover both sides in one step.

The application of each layer, or possibly the set of layers, will be followed by drying, for example by hot or infrared air, possibly seconded by heating rollers. The surface temperature reached will be a minimum of 30 ° C and a maximum of 180 ° C, in relation to the residence time of the coated paper in the heating unit.

Examples

Example 1, not covered by the invention, and the following examples 2 to 3 according to the invention have been made from a fibrous substrate composed of cellulosic cotton fibers. The fibrous substrate is glued at the exit of the paper formation tank by immersion in a PVA bath dissolved with 6% in water in order to deposit 88 g / m2 of PVA in dry weight.

The characteristics of the deposited layers and the properties obtained are detailed in the following tables.

The first (base) layer is deposited by impregnation in glue press. The second (outer) layer is deposited by pencil coating.

Example 1

A sheet comprising the paper substrate defined above is made, which is coated with a bilayer protective coating comprising a base layer based on PU, deposited at a rate of 2.2 g / m2 / face in dry weight, and an outer layer based on PU also deposited at a rate of 5.4 g / m2 / face in dry weight.

The following results are obtained, collected from the following table:

Example 2

A sheet is made comprising the paper substrate defined above, coated with a bilayer protective coating comprising a base layer based on PE, deposited at a rate of 2.4 g / m2 / face in dry weight and an outer layer based on PU deposited at a rate of 4.8 g / m2 / face in dry weight.

The following results are obtained:

Example 3

A sheet is made comprising the paper substrate defined above, coated with a bilayer protective coating comprising a base layer based on PVDC, deposited at a rate of 4.8 g / m2 / face in dry weight and an outer layer based on PU deposited at a rate of 4.8 g / m2 / face in dry weight.

The following results are obtained:

Example 4 (comparative)

A sheet comprising the paper substrate of Examples 1 to 3 is made, which is coated with a layer based on PU in an amount of 2.3 g / m2 / face in dry weight.

The following results are obtained:

Preparation of Examples 1 to 4

The compositions of the formulas PE, PU, PVDC have been given above

Example 5 (comparative)

This is the sheet marketed under the HYBRID reference by the LOUISENTHAL company which comprises a paper substrate laminated with a coating comprising a layer of PU adhesive at 7.0 g / m2 / side, a PET film at 6.0 g / m2 / face and an acrylic varnish at 7.0 g / m2 / face.

The following results are obtained:

Yields

Strength and durability tests

Wash Resistance Tests

The wash resistance test called “low concentration detergent” consists of placing the samples (of dimensions 6.7 cm x 6.7 cm) of paper, printed on chalcography on one side and left to dry for 8 days, in pockets of Stitched fabric, which are placed in a bottle put in rotation for 1 hour at 67 revolution per minute (rpm) at the contact of a water solution at 95 ° C ± 2 ° C containing 750 g of water and 7.5 g of ARIEL® brand powder detergent. Each bottle contains six pockets. After exposure to the detergent, the samples are rinsed and drained with the fingers.

The samples are scored from 0 to 5 (5 corresponding to an absence of reduced printing intensity and 0 to the complete disappearance of the image).

PELIKAN trial

The PELIKAN test consists of soaking two samples (7 cm x 4 cm) in a solution of PELIKAN blue 4001 ink diluted to 10% in demineralized water for 10 min, removing them and placing them between two blotting papers, pressing pressing two round trips with a 10 kg stainless steel roller, let it dry completely and proceed to the evaluation of the AE * colorimetric difference (Elrepho USAV395 white plate measurement in the CIE L * a * b * colorimetric space under D65 illumination and observation at 10 °, according to NF EN ISO 11664-4) before and after exposure to ink on each side.

Dirt resistance test in aqueous phase

It is done:

- a composition of artificial sweat A:

- a powdery colored composition B:

These compounds are mixed using a TURBULA mixer for 30 min in a plastic bottle with six large ceramic balls.

- a fatty composition C:

Be part of a mixture:

Lanoline Lanor TEC of the company LANOLINES DE LA TOSSEE: 200 g

Available TD 0785 from the company NOPCO: 20 g

It is homogenized until the lanolin melts. 28 g of water are added with stirring, then 16 g of Eumulgin BA 10 from the company NOPCO and mixed by heating until a homogeneous preparation is obtained. Allow to cool to room temperature, and 24 g of water are added, mixing for 15 minutes. Then 190 g of water is also added, very slowly at first. A 42% emulsion of lanolin is obtained.

Each sample is previously wrinkled 4 times manually with the IGT wrinkle apparatus.

The samples (75 mm x 140 mm) are placed on wedges so (60 x 35 x 35 mm), around which the samples are kept with three exposed faces.

20 g of fat composition, 10 g of powdery colored composition and 90 ml of composition are arranged artificial sweat and 25 ceramic balls with a maximum diameter of 20 mm in a 2l plastic container. It is stirred and then the four dry wooden wedges are introduced, on which the samples have been placed. The vessel is closed and stirred in a TURBULA mixer at 67 rpm for 20 minutes.

The samples are then rinsed with running water, then dried using two blotting papers, then passed through the oven at 60 ° C for 3 minutes.

The samples are then passed between two blotters with an iron in the 2/3 position.

The samples will be calendered one by one with one step per side, at 15 bar pressure, minimum speed.

The dirt index U is then evaluated:

where Bi is the whiteness measured on the samples after the test, Bo is the whiteness before the test, Ji is the degree of yellowing after the test and Jo is the degree of yellowing before the test. The whiteness is an average value of three measurements according to NF ISO 2470-26167 and the degree of yellowing is an average value of three measurements (measurement with the DATACOLOR ELREPHO 3000 device of the LORENTZEN & WETTRE company, current in the paper field ).

Alumina balls should not weigh less than 300 g. They are conditioned before use by placing them in the TURBULA with an abrasive cream and a little water for 1.5h.

Abrasion of printed inks

Composition of artificial sweat: A. whose formula was given above.

Abrasive composition: 2D Vermiculite from the company EFISOL.

Fat composition: Same formula as C above.

The samples are printed with an S12W blue ink from the SICPA company allowed to polymerize for 8 days before the test, the printing being done in chalcography.

The samples are 75 x 140 mm strips.

20 g of fat composition, 10 g of abrasive composition and 90 ml of artificial sweat and 25 ceramic balls with a maximum diameter of 20 mm are placed in a 2l plastic bottle.

After cooling, the samples are placed on four wooden wedges as in the previous test. The set is rotated for 20 minutes at 76 rpm.

The rinsing and drying of the samples are carried out as described in the previous test.

The results are evaluated with a visual scale ranging from 1 to 5 (1 severely degraded sample, 5 absence of deterioration).

Powder Dirt Test

equipment

- metal drum (23cm x 23cm x 33cm) with lid

-2 kg of 2.5 mm diameter glass balls

- rubber with male and female threading

- Drop count

- drill

- Clean cloth

- beaker

Composition of the dirt mix

Sample Preparation

- cut samples in banknote format (long direction of travel)

- drill a hole in each corner approximately 9 mm from each angle

- place a rubber weight on each corner and press tightly.

Preparation of the dirt mixture

- mix 0.3 g of kaolin with 2000 g of glass balls

- add 0.4 g of olive oil, soybean oil and ethanol

- mix for 5 min in the metal container at 60 rpm.

Process

- add 20 samples in the metal container

- rotate at 60 rpm (metal drum horizontally) for 15 minutes in one direction and then in the other - take samples, remove rubber weights

- drain three times with a damp cloth and then once with a dry cloth each face

- proceed with the evaluation by calculating the colorimetric difference AE * (Elrepho USAV395 white plate measurement in the CIE L * a * b * colorimetric space under D65 illumination and observation at 10 °, according to NF EN ISO 11664-4 ) from measurements on samples before and after the test.

Measurements are made on five sample sites.

Very good results are observed with the papers according to the invention in the three tests "Abrasion of printed inks", "dirt in aqueous phase" and "powdery dirt".

The invention is not limited to the examples illustrated.

The expression "comprising a" is synonymous with "comprising at least one."

Claims (14)

1. High durability paper sheet for the manufacture of banknotes, comprising: - a fibrous substrate, - a protective coating that completely covers one face at least of the fibrous substrate, this coating comprising at least one base layer of the substrate side and a non-fluorinated external printable layer based on polyurethane, which covers the base layer, comprising the base layer a polyolefin or PVDC. 2. Sheet according to claim 1, the total thickness of the protective coating being greater than or equal to 3 pm, especially between 5 and 25 pm. 3. Sheet according to claim 1, 2 or 3, the external printability layer comprising a mineral and / or polymeric particles charge, especially a mass amount, in dry weight, less than 50%, better between 5% and 25%, better still between 6% and 10%, within the external printability layer. 4. Sheet according to claim 4, the external printability layer comprising a mineral filler comprising silica and / or kaolin and / or talc and / or calcium carbonate. 5. Sheet according to any one of the preceding claims, the external printability layer comprising at least 25% dry weight of polyurethane, preferably at least 30% dry weight of polyurethane, better at least 50% dry weight of polyurethane. 6. Sheet according to any one of claims 1 to 4, the external printability layer comprising between 25% and 50% polyurethane in dry weight, and preferably an acrylic compound mixed with the polyurethane, especially in substantially equal proportions. 7. Sheet according to any one of the preceding claims, the base layer comprising a PE, better a modified PE comprising acid groups, and preferably carboxylic acid groups, or a fluorinated polymer, in particular PVDF, or acrylic. 8. Sheet according to any one of the preceding claims, incorporating the fibrous substrate at least one security element. 9. Method of manufacturing a high durability sheet for banknote, which comprises the step which consists in coating a base layer in a fluid state on a fibrous substrate and coating in line or out of line on the layer of base a non-fluorinated external printable layer based on polyurethane, especially with a proportion of polyurethane between 25 and 50% by dry weight, preferably mixed with an acrylic compound, the base layer comprising a polyolefin or a PVDC, preferably a polyolefin, better a PE, even better a PE modified by acid groups, preferably carboxylic acid groups. 10. Process for manufacturing a high durability sheet according to the preceding claim, the fibrous substrate being glued prior to depositing the base layer. 11. Method according to one of claims 9 or 10, the base layer being deposited in line with the help of an impregnator. 12. The method according to one of claims 9 or 11, the fibrous substrate being glued prior to its passage in the impregnator. 13. The method according to claim 10 or the preceding claim, the gluing being carried out with PVA. 14. The method according to any one of claims 9 to 13, the coating of the external in-line printability layer being carried out.